L920 
Jopy 1 



THE INHERITANCE OF SALMON 
SILK COLOR IN MAIZE 



A THESIS 

Presented to the Faculty of the Graduate School 
OF Cornell University for the degree of 

DOCTOR OF PHILOSOPHY 



BY 

ERNEST GUSTAF ANDERSON 



Published as Cornell University Agricultural Experiment Station 
Memoir 48— November, 1921 



■--.I 



THE INHERITANCE OF SALMON 
SILK COLOR IN MAIZE 



A THESIS *^^^ 

Presented to the Faculty of the Graduate School 
OF Cornell University for the degree of 

DOCTOR OF PHILOSOPHY 



BY 



ERNEST GUSTAF ANDERSON 



Published as Cornell University Agricultural Experiment Station 
Memoir 48 — November, 1921 



"B, 






02 



CONTENTS 

PAGE 

Nomenclature 539 

Description of silk colors 540 

Preliminary studies and indications 541 

Analysis of inheritance 542 

Interrelations of green, salmon, and brown silks 542 

Relation of salmon silks to pericarp color 542 

Relation of salmon silks to the B and PI factors for plant color. . 545 

Relation of salmon silks to the A factor 546 

Relation of salmon silks to the R factor 546 

Summary of inheritance 547 

Linkage relations of Y, PI, and Sm 547 

Preliminary tests of linkage of PI and Sm 547 

Construction and results of three-point tests 548 

The chromosome map 549 

Coincidence of crossing-over 553 

Summary of linkage studies 554 

Literature cited 554 



535 



THE INHERITANCE OF SALMON SILK COLOR IN MAIZE 



THE INHERITANCE OF SALMON SILK COLOR IN MAIZE ^ 

E. G. Anderson 

At the Nebraska State Corn Show of 1908, a number of odd types of 
corn were gathered together to form a, "freak" class. Among them was 
a " Bronze Pop Corn," so named became of a Kght bronze color in 
the pericarp. This ear was obtained by Professor R. A. Emerson, after 
the exhibit was over, in order to study the inheritance of that pericarp 
color. The plants grown therefrom were also characterized by brown or 
brownish silks (Plate LIII). An outcross gave green silks in the Fi. From 
brown-silked segregates in the progeny of this cross, a true-breeding stock 
was again obtained. This stock was used in crosses for a study of the 
inheritance of pericarp color. In one small F2 of only five plants, there 
appeared three with green silks and one with brown. The fifth plant 
had very brilliant salmon or orange-colored silks (Plate LII). This plant 
was a dilute sun red with red pericarp. It was crossed with red, green, 
and brown silk colors, and with a purple plant having brown silks. Fi's 
were grown and selfed to obtain F2 progenies. The crosses with red and 
with green silks gave in Fi red and green silks, respectively. The cross 
with brown silks gave salmon. 

In order to devote more time to studies on aleurone and plant colors 
and other problems. Dr. Emerson at this point requested the writer to 
take up the study of these silk colors and their relation to other characters 
in maize. In his further studies the writer has had the advantage of the 
hearty cooperation and ever-ready suggestions of Dr. Emerson, and he 
wishes to acknowledge his sincere gratitude for this help and encouragement. 

NOMENCLATURE 

The factors referred to in this paper, together with the factor symbols 
used, are given in the following list: 
A a — Anthocyanin pigment. A factor pair for pigmentation of aleurone, 

sheaths, leaves, anthers, and so forth. (Emerson, 1918, 1921.) 
B b — ■ Brown plant color. A factor pair for leaf and sheath pigmentation. 

(Emerson, 1921.) 

1 Paper No. 83, Department of Plant Breeding, Cornell University, Ithaca, New York. 

539 



540 E. G. Anderson 

PI pi — Tnrple anthers. A factor pair for pigmentation of anthers 
sheaths, pericarp, and so forth. (Emerson, 1921.) 
These three factor pairs interact to give the following plant color types 
described by Emerson (1921) : 

A B PI — Purple 

A B pi — Sun red 

A h PI — Dilute purple 

A h pi — Dilute sun red 

a B PI — Brown 

a B pl — Green 

a h PI — Green 

a h pl — Green 

^r ^0 ^r ^ ^ch — jj^g^ aleurone. A series of allelomorphs affecting antho- 
cyanin pigmentation in aleurone, sheaths, leaves, pericarp, anthers, 
and silks. (East and Hayes, 1911; Emerson, 1918, 1921.) 

Pp — Pericarp color. Two of a series of allelomorphs for pericarp 
coloration. (Emerson, 1911.) The bronze type was so pale in 
color that it could not be satisfactorily distinguished from white 
(colorless). Only two symbols are used herein, P for red pericarp 
and p for white or bronze pericarp. 

Yy — Yellow endosperm. (East and Hayes, 1911; Emerson, 1921.) 

Sm sm — Salmon silk color. Described in this paper. 

DESCRIPTION OF SILK COLORS 

The colors of silk in maize may be described as follows: 

1. Green (Plate L). Silks light green, paler below husks; varying 
from a pure pale green to yellowish green. 

2. Red (Plate LI). Silks green, as above, with the addition of a red 
anthocyanin pigment where exposed to light. The amount of red pig- 
ment may vary from a slight trace in the hairs, to sufficient to obscure 
the green color, giving the silks a deep or dark red color. The darker red 
silks frequently have some red below the husks. Emerson (1921) has 
shown this color to be due to the R factor. Microscopic sections show 
anthocyanin pigment in peripheral parts of the silks ^ 

2 The microscopic sections are prepared as follows: Pigmented tissue is fixed for from twelve to 
twenty-four hours in a saturated solution of mercuric chloride in 95-per-cent alcohol, and washed with 
95-per-cent alcohol without iodine. The usual paraffin method of embedding and sectioning is followed 
and the preparations are mounted in balsam without staining. Sections from 15 to 25 micra in thickness 
have proved satisfactory. 



Memoir 48 



Plate 1 




GREEN SILKS 

Silks of this type may be associated with any plant-color type. Purple husks are shown 
here for contrast 

(Drawing by Carrie M. Preston) 



Memoir 48 



Plate LI 





RED SILKS 

The red pigment develops only in the parts of the silks exposed to the light 
(Drawing by Carrie M. Preston) 



Memoir 48 



Plate Lll 



7 




SALMON SILKS 

The color develops beneath the husks as well as in exposed parts of the silks 
(Drawing by Bernice M. Branson) 



Memoir 48 



Plate LIII 





W||^^MHr 



BROWN SILKS 

The color develops in both exposed and protected parts of the silks 
(Drawing by Carrie M. Preston) 



Inheritance of Salmon Silk Color in Maize 541 

3. Salmon (Plate LII). Silks light salmon-orange to salmon. The 
color below the husks is similar to that of exposed parts. Microscopic 
sections show only a faint brownish cast to the tissues thruout the 
silks. 

4. Brown (Plate LIII). Silks orange-pink to pale salmon or salmon-buff 
in both exposed and covered parts. Salmon and brown silks intergrade, 
forming a continuous series. The lighter forms are difficult to distinguish 
from the yellowish green silks of No. 1. Both salmon and brown silks 
may have red anthocyanin pigment present, as in No. 2. 

preliminary studies and indications 

Previous tests had shown salmon silk color to be recessive to green and 
at least partially dominant to brown. Crosses of salmon and brown did 
not give green color in the Fi. This was taken to indicate that these 
colors were recessive for a common factor. The anthocyanin pigment 
present in red silks was shown to be inherited separately by the occurrence 
of all combination classes (green-red, green, salmon-red, and salmon) 
in the F2 of red x salmon. 

From observation of cultures previously grown, both salmon and brown 
silks were known to occur on dilute sun red, on sun red, and on purple 
plants. Their occurrence on brown or green plants had not been recorded. 
Microscopic examination of the pigments of maize had shown the presence 
of a purple-red anthocyanin pigment in purple, sun red, dilute purple, 
and dilute sun red plants. When the A factor is recessive, no anthocyanin 
develops (except traces in the shank and the inner husks of brown plants) . 
Instead, a yellow or brownish pigment may be formed. A similar rela- 
tion holds with the pericarp pigments. Red pericarp color is due to an 
orange-red or brick-red pigment. Its homolog with recessive A is yellow- 
ish brown, similar in appearance to the brown plant-color pigment. 
The quantity of pigment in salmon silks is so small that microscopic 
sections gave little information. But the color of the salmon silks was 
so similar to the color of thin sections of red pericarp as to suggest the 
possible identity of the pigments. The brown silk might, it was thought, 
bo only a dilute form of salmon. These suggestions were further strength- 
ened by the fact that the original salmon-silked plant had red pericarp 
and that the brown silks had been obtained from a plant with bronze 



542 E. G. Anderson 

pericarp. Sections of this bronze pericarp showed a small amount of 
orange pigment. 

With these suggestions in mind as a working basis, experiments were 
planned to test them. 

ANALYSIS OF INHERITANCE 

Interrelations of green, salmon, and brown silks 

The indications just mentioned, regarding the relationships of these 
silk colors, were all checked and corroborated by further tests. Crosses 
of green with salmon gave green in the first generation, segregating green 
and salmon, or green, salmon, and brown, in the second. The distinction 
between salmon and brown was not sharp. With the small numbers used 
in these tests, the numbers approach a simple ratio of 3 greens to 1 salmon 
or to 1 salmon and brown. 

Crosses between green and brown likewise gave green, segregating in 
the r2 into green and brown or in some cases into green, salmon, and brown. 
In either case there was about 75 per cent of greens. 

Crosses of salmon with brown gave salmon. The F2 ranged from salmon 
to brown, with salmon predominating. 

These results show that there is a conomon factor pair which differen- 
tiates between green on the one hand and salmon and brown on the other. 
This pair is herein referred to as the sahnon-silk factor pair and is desig- 
nated by the symbols Sm sm. 

The difference between sahnon and brown silks is not explained by these 
simple tests, tho the occurrence of brown silks in the progenies of out- 
crosses of salmon, and vice versa, is at least a strong indication of one or 
more modifying factors. 

Relation of salmon silks to pericarp color 

In order to test the relationship of the salmon factor to the factor for 
pericarp color, two series of crosses were made. In the first, a colored- 
pericarp, green-silked plant (P Sm) was crossed with a light-bronze-peri- 
carp, brown-silked plant (p sm). In the second, two white-pericarp 
green-silked plants (p Sm) were crossed with the original salmon-silked 
plant, which had red pericarp (P sm). The Fi's were crossed with. the 
double recessive. The results are given in table 1 : 



Inheritance of Salmon Silk Color in Maize 543 

TABLE 1. Relation of Salmon Silks to Pericarp Color 



I. Backcrosses of P Sm x p sm with p sm 



Pedigree no. 


PSm 


P sm 


p Sm 


P S771 


Total 


253-4 


9 

5 

9 

41 


6 
11 

G 
32 


11 
12 
15 
41 


20 

10 

7 

39 




255 




256 




777-8 








Totals 


64 


55 


79 


76 


274 



Observed per cent of recombinations 48.9 

Per cent expected with independent segregation 50.0±2.0 



II. Backcrosses of p Sm x P sm with p sm 



Pedigree no. 


PSm 


P S771 


p Sm 


p sm 


Total 


238-9 


43 
31 
23 
43 


61 
28 
25 
40 


51 
31 
25 
50 


53 

38 
28 
40 




241-2 




779-80 

781-2 . 








Totals 


140 


154 


157 


159 


610 



Observed per cent of recombinations 48 . 9 

Per cent expected with independent segregation 50. Oil .4 



The observed per cent of recombinations is 48.9 in both series, which 
is in very close agreement with the expectancy for independent segrega- 
tion of P and Sm. 

In order to determine the possible relation of pericarp color to the dif- 
ference between salmon and brown silks, a light-pericarp, brown-silked 
plant was crossed with a red-pericarp salmon. This was backcrossed 



544 



E. G. Anderson 



with a light-pericarp, brown-silked plant similar to the one parent. The 
silk colors were noted during the summer. It was impossible to make 
any sharp separations, for the colors varied from a deep salmon to a typical 
or even light brown. The presence of red anthocyanin pigment added to 
the difficulty, as did also the fact that the silks could not be noted at the 
same stage. So they were only roughly classified, the classifications 
from time to time not being entirely comparable. The notes were under- 
scored for a nmnber of good saknons and browns. The pericarp colors 
were determined in the fall. The results are given in table 2: 

TABLE 2. Backcrosses of p, sm x P sm with p sm 



Silk color 



Salmon, underscored 

Salmon 

Salmon — 

Salmon-brown 

Brown-salmon 

Brown + 

Brown 

Brown, underscored. 



Red 


White or 


pericarp 


light 
bronze 


17 





167 


19 


25 


10 


33 


11 


10 


24 


24 


65 


11 


172 





44 



It will be seen from this table that most of the red-pericarp plants had 
been noted as having salmon silks, while the light-pericarp ones were 
mostly noted as having brown silks. It is also significant that, of those 
cases in which salmon was underscored, all had red pericarp. Like- 
wise, of the cases in which brown was underscored, all had light pericarp. 
Since the salmon factor pair Sm sm has been shown to segregate inde- 
pendently of the factor pair P p for pericarp color, this variation can- 
not be due to the Sm sm pair. The conclusion is drawn that the intensity 
of pigmentation of silks recessive for sm is largely a function of the inten- 
sity of pigmentation of the pericarp or of some factor closely associated 
with the factor for pericarp color. The former view is substantiated 



Inheritance of Salmon Silk Colou in Maize 



545 



by the fact that no selfed progenies from light-pericarp plants have ever 
given any good sabnon silks, while progenies from red-pericarp plants 
have always given some salmon silks even tho the parents had been noted 
otherwise. No brown silks have been found in families breeding true 
for red pericarp. 

Relation of salmon silks to the B and PI factors for -plant color 

Several crosses made with salmon silks involved the B factor. Both 
Fi combinations, B Sm x b sni and B sm x b Sm, were backcrossed with 
the double recessive. The results (table 3) show independent segregation 
of these factors. 

TABLE 3, Relation of Salmon Silks to the B and PI Factors for Plant Color 
I. Backcrosses of B Sm x b sm with b sm 



Pedigree no. 


B Sm 


B sm 


bSin 


b sm 


Total 


241-2 


25 
21 


24 
19 


39 

28 


41 
35 




779-80 








Totals 


46 


43 


67 


76 


232 



Observed per cent of recombinations 47.4 

Per cent expected with independent segregation 50.0 ±2. 2 



II. Backcrosses oi B sm xb Sm with b sm 



Pedigree no. 


B Sm 


B sm 


b Sm 


b sm 


Total 


774-6 


78 


62 


80 


53 


273 



Observed per cent of recombinations 47 . 6 

Per cent expected with independent segregation 50.0±2.0 



546 E. G. Anderson 

One of the crosses of the original salmon-silked plant, A h pi sm, was 
with a purple plant with green silks, A B PI Sm, related to the bronze 
stock. The progeny consisted of purple and sun red plants with green and 
salmon silks, showing the parent to have been heterozygous for both 
PI and Sm. Two small plantings gave the following distributions; 
PI Sm, 26; PI sm, 7; pi Sm, 4; pi sm, 23; whereas equality of the four classes 
would be expected if the factors were independent. This was obviously 
a linkage relation. The factor PI was known to be linked with a factor 
Y for yellow endosperm (Emerson, 1921). Tests of the linkage relations 
within this group are given in a later section of this paper. 

Relation of salmon silks to the A factor 

From an outcross of the original salmon-silked plant with one heterozy- 
gous for brown silks and for the A factor, several plants were selfed. One 
sun red plant was homozygous recessive for the salmon silk factor and 
heterozygous for A and B. Thirty-four sun red and dilute sun red plants 
had salmon or brown silks. Two others were first noted as green but were 
presumably a light brown, both having white pericarp. Eleven green 
plants appeared, all having green silks. Later observations on green and 
brown plants of other families segregating for both a and sm have like- 
wise failed to reveal any green plants with other than green silks. That 
this is not due to linkage is shown by the linkage of Sm with the PI factor, 
which is known to be independent of A, and by the fact that the green 
plants have green silks in families that are homozygous recessive sm. 

Relation of salmon silks to the R factor 

Two questions of interest arose regarding the relation of salmon silk 
color to the R series of allelomorphs. The first was the relation of cherry 
pericarp color to the intensity of color in salmon or brown silks; the second 
was the possibility of the occurrence of salmon silks on green plants of 
the constitution R" A h PI or R^ A b pi. 

To test the effect of cherry pericarp, a sun red with brown silks, 
A B pi sm /, was crossed with a dilute purple with cherry pericarp and 
green silks, A h PI Sm f . Backcrosses gave a few plants with cherry 
pericarp and brown silks. They were not noticeably different in silk 
color from the white-pericarp plants of the same families. 



Inheritance of Salmon Silk Color in Maize 547 

To test for the occurrence of salmon silks on green plants of the consti- 
tution R" A h PI or R^ A b pi, a dilute purple plant with sahnon silks 
was crossed with a green plant of the constitution R^ A b pi. Two of 
the Fi plants were selfed. Purple seeds only were planted. These 
gave 44 dilute purples and dilute sun reds, of which 32 had green silks and 
12 had salmon. There were 25 green plants, R" A b, 18 of which had green 
silks; the other 7 had typical salmon and brown silks. 

From these two tests, it may be concluded that salmon silk color is 
not dependent on the R factor nor is it noticeably influenced thereby. 
This is similar to the relation between red pericarp color and R" (Emerson, 
1921). 

Summary of inheritance 

Salmon and brown silks are recessive to green silks by a single factor 
pair, Sm sm. 

This factor, Sm, is independent in inheritance from P (pericarp), 
A (aleurone and plant color), B (plant color), and R (aleurone, plant color, 
cherry pericarp, and red silk color). 

It is linked with the factor PI (plant color), and consequently also with 
Y (yellow endosperm). 

Dominant A is necessary for the production of salmon or brown silk 
color; that is, the combination a a sw sm is green. 

The intensity of pigmentation of salmon-brown silks is directly related 
to the intensity of pigmentation of the pericarp. 

The relation of the factors A , Sm, and P to silk color may be represented 
schematically as follows: 

A Sm P = Green a Sm P = Green 

A Sm p = Green a Sm p = Green 

A sm P = Salmon a sm P= Green 

A sm p = Brown a sm p = Green 

LINKAGE RELATIONS OF Y, PI, AND Sm 

Preliminary tests of linkage of PI and Sm 

The first indication of the linkage of the Sm and PI factors was observed 
in the progeny of an outcross of the original salmon with a purple plant 
having green silks. This plant proved to be heterozygous for both Sm 



548 E. G. Anderson 

and PL To the distribution given on page 546 may be added the data 

from a dupHcate planting by Dr. Emerson: 

Per cent of 
crossing- 
Pi Sm PI sm pi Sm pi sm over 

111-2 26 7 4 23 

From Emerson 25 9 4 38 



51 1(3 8 61 17.6 



Two other backcrosses were then made, which the following year gave 
the results: 



238-9 
241 . . 









Per cent of 


PlSm 


PI sm 


pi Sm 


crossing- 
pl sm over 


76 


24 


20 


93 20.3 


60 


4 


4 


66 6.3 



Construction and results of three-point tests 

In the meantime, crosses were made to involve the Y factor for yellow 
endosperm in addition to PI and Sm, since Y and PI were known to be 
linked. To get a satisfactory three-point backcross test involved several 
difficulties, as follows: 

1. Yellow endosperm is not easily distinguished from white if brought 
in only by the pollen. This is assumed to be due to the dominant F's 
being represented only once in the triple-fusion endosperm nucleus. It is 
therefore desirable that the Fi plants should be used as female parents 
in the backcrosses. 

2. Brown silks are not readily separated from green. This difficulty 
can be avoided only by having red pericarp in each plant. But the 
presence of red pericarp obscures the color of the endosperm. So in order 
to make endosperm separations possible, the female parent of the back- 
cross must be free from red pericarp. 

3. Purple and dilute purple plants usually have some purplish pigment 
in the pericarp, which in some cases interferes with the classification of 
yellow endosperm. 



Inheritance of Salmon Silk Color in Maize 549 

4. The dominant A factor must be present in every individual where 
silk color separations are to be made. 

5. Aleurone color must be avoided. 

6. Presence of the dominant B factor, while not affecting accuracy, 
would nevertheless facilitate note-taking by making all the plants of 
two sharply differentiated classes, purple and sun red. 

To avoid as many as possible of these difficulties and accomplish the 
results within the shortest period of years, the following procedure was 
put into effect: Crosses were made involving the factors Y, PI, and Sm 
in different combinations. In all of these crosses, pericarp color and also 
the R factor for aleurone color were kept recessive. At the same time, an 
attempt was made to find or isolate a stock of the triple recessive of the 
desired composition. Tests of all available salmon-silk material revealed 
two closely related families breeding true for red pericarp, white endosperm, 
and recessive r. Both families consisted of sun red and dilute sun red 
plants showing the B factor to have been heterozygous. These were used 
the following year in the backcrosses. Their composition was y y pi pi sm 
sm r r P P A A, some plants being homozygous and some heterozygous 
for dominant B. Pollen of these plants was used on silks of the Fi 
crosses. 

These backcrosses were made in 1918 and the progenies were grown in 
1919. The results are given in table 4 (page 550). The percentages of 
crossing-over are: Y-Pl, 28.9; Pl-Sm, 9.1; Y-Sm, 36.6; showing their 
relative order to be Y- Pl-Sm. 

While material for these tests was being built up, some much less satis- 
factory backcrosses were made by pollinating white-endosperm, brown- 
silked, dilute sun red plants with pollen from crosses involving Y, PI, 
and Sm. These were grown in 1918. The results are given in table 5 
(page 551). 

A summary of the percentages of crossing-over is given in table 6 
(page 552). 

The chromosome map 

From the totals of all the data obtained on these linkage relations, the 
observed percentages of crossing-over are found to be 29.70 for Y-Pl, 
10.01 for Pl-Sm, and 36.79 for Y-Sm. This shows their relative map 
order to be Y-Pl-Sm. The distance from Y to PI as observed is 29.7, 



550 



E. G. Anderson 



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Inheritance of Salmon Silk Color in Maize 



551 



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552 E. G. Anderson 

TABLE 6. Summary of Linkage Data 






Pedigree no. 


Total 

number 

of 

plants 


Percentage of crossing-over 




Y-Pl 


Pl-Sm 


Y-Sm 


111-2 


136 
213 
134 




17.5 

20.7 

6.0 




238-9 




241 








Totals 


483 




15.73 








772-3 


219 
273 
177 
158 
103 


36.1 
34.8 

28.2 
27.2 
34 


10.5 

9 5 

10,2 

8.9 
12.6 


40.2 


774-6 


40.7 


781-2 


31.6 


777-8 


34.8 


779-80 


36.9 






Totals 1918 


930 


32.47 


10.11 


37.42 






1451-3 


287 
98 
496 
413 
376 
187 
318 
355 
143 
236 
184 


37.7 

37.8 
31.0 
30.0 
26.3 
19.3 
23.6 
34.1 
25.9 
22.0 
27.2 


10.1 

6.1 

7.5 

8.7 

7.4 

6.4 

6.3 

7.6 

13.3 

14.0 

18.5 


45.6 
41.8 
38.5 
36.8 
32.7 
25.7 
29.9 
41.1 
37.8 
35.2 
37.0 


1454r-6 




1460-2 




1467-8 




1471-3 


1476-7. . 


1478-80 . . 






Totals 1919 . 


3,093 


28.87 


9 09 


36.60 




Totals 1918-1919 


4,023 
4,506 


29.70 


9 32 
10.01 


36.79 









or approximately 30 units. Since in such long distances double crossing- 
over may be expected, a corrected map distance should be 30 plus twice 
the per cent of unobserved double crossovers between the two points. 
But with the high amount of interference indicated by the small number 
of observed coincident crossovers in the two regions Y-Pl and Pl-Sm, the 
corrected value for these data is probably not much above 30 or 35. The 
value 10 for the map distance between PI and Sm is probably correct 
for these data. 



Inheritance of Salmon Silk Color in Maize 553 

It should be understood that the chromosome map is primarily a graphic 
representation of the data on linkage relationships. Its correspondence 
with actual positions on the chromosome itself is not implied, tho the 
work of Morgan and his coworkers has given much evidence of at least a 
correspondence between relative map order and the actual relative position 
of the genes in the chromosome. 

The variability of the percentages of crossing-over shown in table 6 
is not greater than would be expected of heterogeneous data. Gowen 
(19r9) has shown crossing-over in Drosophila to be an extremely variable 
phenomenon. Plough (1917) has shown it to be modified by temperature, 
and Bridges (1915) by age of the individual. The subject of variation 
of crossing-over in maize must remain for study with less difficult characters 
than those involved in these experiments. 

The distributions when the Fi's were used as pistillate and as staminate 
parents give nearly the same averages, but the data are inadequate for 
any conclusion except that the crossing-over is not widely different in 
the two cases. 

Coincidence of crossing-over 

Coincidence of crossing-over in two regions of a chromosome is the 
ratio of observed coincident (simultaneous) crossing-over to the calculated 
expectancy. The expectancy is the product of the percentages of crossing- 
over of the two regions. The actual calculation may be simplified, as 
shown by Weinstein (1918). The derived formula is 

Coincidence = — 
ab 

in which n = the total number of individuals, 

X = the number of coincident crossovers, 
a and b = the total number of crossovers in the respective regions. 

The coincidence of crossing-over in the two regions Y-Pl and Pl-Sm, 
calculated from tables 4 and 5, is as follows: 

From table 4 : 



From table 5 : 



^ . . , 21 X 3093 „ _ 

Coincidence = = 0.26 

893 X 281 



^ •• , 24 X 930 ^ ^^ 

Coincidence = = 0.79 

302 X 94 



554 E. G. Anderson 

From combined data of tables 4 and 5: 

^ . .- 45x4023 „,„ 

Comcidence = = U.4U 

1195 X 375 

These values are entirely comparable with those listed by Weinstein 
(1918) for Drosophila. From this and the similarity of all phases of 
linkage and crossing-over, it is evident that the mechanism of crossing- 
over in maize is not strikingly different from that in Drosophila except 
in one respect. In Drosophila, crossing-over occurs in oogenesis only, 
in spermatogenesis not at all. In maize the phenomena of crossing-over 
are at least of the same order in both megasporogenesis and microsporo- 

genesis. 

Summary of linkage studies 

The factor Sm for salmon silk color is shown to be linked with the 
factor Y for yellow endosperm and the factor PI for plant and anther color. 

The relative order of these three factors is Y-Pl-Sm. 

The amount of crossing-over between Y and PI is about 30 per cent; 
between PI and Sm it is about 10 per cent. 

The observed coincidence of crossing-over in the two regions Y-Pl and 

Pl-Sm was about 0.4. 

LITERATURE CITED 

Bridges, Calvin B. A linkage variation in Drosophila. Journ. exp. 

zool. 19:1-21. 1915. 
East, E. M., and Hayes, H. K. Inheritance in maize. Connecticut 

Agr. Exp. Sta. Bui. 167:1-142. 1911. 
Emerson, R. A. Genetic correlation and spurious allelomorphism in 

maize. Nebraska Agr. Exp. Sta. Ann. rept. 24:58-90. 1911. 
A fifth pair of factors, A a, for aleurone color in maize, and 

its relation to the C c and R r pairs. Cornell Univ. Agr. Exp. Sta. 
« Memoir 16:225-289. 1918. 

The genetic relations of plant colors in maize. Cornell Univ. 



Agr. Exp. Sta. Memoir 39 : 1-156. 1921. 

GowEN, John Whittemore. A biometrical study of crossing over. On 
the mechanism of crossing over in the third chromosome of Droso- 
phila melanogaster. Genetics 4:205-250. 1919. 

Plough, Harold H. The effect of temperature on crossingover in 
Drosophila. Journ. exp. zool. 24:147-209. 1917. 

Weinstein, Alexander. Coincidence of crossing over in Drosophila 
melanogaster {ampelophila) . Genetics 3 : 135-172. 1918. 



Memoir 41 , Lysimeter Experiments — 11. the seventh preceding number in this series of publications, was 
mailed on November 16, 1921. 



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